This invention relates to an epoxy resin composition which is suited for use in encapsulating a semiconductor element, and to a resin-encapsulated semiconductor device which is encapsulated (sealed) with the epoxy resin composition.
Epoxy resin is extensively employed as a sealing (encapsulating) material for various devices such as a transistor, an IC and an LSI, because epoxy resin is relatively cheap, high in productivity, and well balanced in various properties such as electric property, moisture resistance, heat resistance, mechanical property and adhesivity with an insert.
With the current trend to further increase the integration of semiconductor elements, a semiconductor chip is now increasingly enlarged and hence the package of resin-encapsulated semiconductor device is also increasingly enlarged. On the other hand, with an increasing miniaturization of mounting space, the package is now being made thinner.
In order to meet the demand for a high density packaging of electronic parts or for an automatization of assembling process, a surface mounting system is now becoming most popular as a mounting method of semiconductor devices in place of the conventional insertion system. As for a method of mounting a semiconductor package on a wiring circuit board according to this surface mounting system, various processes such as an infra-red ray reflow, a vapor phase reflow and a solder dipping have been adopted.
In any of these mounting processes, the package is entirely exposed to a high temperature in the order of 200 to 260.degree. C., so that moisture absorbed in the resin encapsulating a semiconductor device may be suddenly expanded, thus giving rise to various problems such as the generation of a crack in the sealing resin or the peeling off of the sealing resin at an interface thereof with the semiconductor device, whereby badly deteriorating the reliability of the resin-encapsulated semiconductor device. Moreover, there have been observed various problem also in a semiconductor device sealed with a resin when the package is exposed to high temperatures, e.g. the passivation film protecting a wiring layer made of Al for instance may be cracked or a bonding wire may be disconnected.
In order to avoid these problems, the epoxy resin to be employed for sealing a semiconductor element is required to be low in stress intensity and excellent in solder reflow resistance. Furthermore, in order to meet the aforementioned trends of the enlargement of chip and of the thinning of package, the sealing resin is further required to be high in fluidity and excellent in moldability.
The low stress intensity and the solder reflow resistance can be improved by filling a filler in high density so as to lower the thermal expansion coefficient or water absorbency of the resin. In this case however, the melt viscosity of the resin would be greatly increased due to a high filling ratio of the filler, thus lowering the fluidity of the resin. With a view to solve this problem, there have been proposed various ideas, i.e. a method of employing a composition where fillers having various particle diameters are mixed with a resin (Japanese Patent Unexamined Publications H/5-230188, H/5-239190 and H/5-239321); and a method of employing a low viscosity bifunctional epoxy resin having a phenyl skeleton or a bisphenol skeleton in place of employing the conventional cresol novolak type epoxy resin.
However, when a semiconductor element is sealed with a lower-viscosity epoxy resin including aforementioned fillers, it is difficult to prevent the generation of void or burr due to the low viscosity of the resin composition. If the void volume of resin tablet is reduced by increasing the tableting density of the resin tablet (or the density of tablet) for sealing a semiconductor device, the generation of void may be inhibited. Accordingly, a method has been employed to pulverize the particle diameter of epoxy resin molding material as small as possible before tableting. On the other hand, the molding resin powder formed of the aforementioned low viscosity epoxy resin is so low in softening point that it can be easily blocked at room temperature and is poor in workability for forming tablet. At the same time, the employment of this low viscosity epoxy resin invites new problems such as the lowering of tableting density of the resin and the lowering of fluidity of tablet.
Furthermore, since the resin composition is low in viscosity, the resultant resin tablets tend to adhere to each other when they are stored for a long period of time. When resin tablets are adhered to each other, the transfer failure of the tablets may be caused at the occasion of automatic molding process. If the resin tablets adhered to each other in this manner are forcibly separated from each other, the fracture or deformation of the tablets may be caused, thus leading to the generation of voids after molding.
As mentioned above, it is difficult to avoid the generation of voids and burr if an epoxy resin sealing material which is excellent in fluidity and low in viscosity is employed for encapsulating a semiconductor device, and, because of this, it has been impossible to obtain an encapsulated semiconductor device having a sufficient reliability. Furthermore, a sealing material consisting of such an epoxy resin composition is accompanied with various problems when it is formed into powder or tablets.